The present invention relates to a reflection type of catoptric optical system, and is suitable, for example, for a projection optical system that projects light from an imaging device, such as a liquid crystal panel and a digital micromirror device (“DMD”) manufactured by Texas Instruments Incorporated, onto an object surface, such as a screen.
Prior art has proposed to shorten an optical path length in an optical system for a compact catoptric system that includes plural optical reflective surfaces.
Japanese Patent Application, Publication No. 2001-222063 has proposed a compact rear projection optical system that uses three curved mirrors to properly extend a screen diagonal length relative to an optical path length, and enables two plane mirrors to easily return or turn around the light.
Japanese Patent Application, Publication No. 2001-221949 has proposed a thin and compact oblique incidence optical system that uses four reflective surfaces to properly adjust a ratio between a distance from a final reflective surface to a screen and a distance from a final reflective surface to a lens system, maintains an optical path length for turnaround of the light, and arranges a return mirror.
Another proposed technology has arranged a curved mirror on an optical path to prevent an expansion of the optical path in the optical system for a compact structure.
For example, Japanese Patent Application, Publication No. 2001-242381 sets a spherical mirror just before a screen in addition to a three- or four-aspheric mirror system to produce a rear projection monitor catoptric imaging optical system that includes totally four or five mirrors.
Japanese Patent Application, Publication No. 07-013157 (corresponding to U.S. Pat. No. 5,477,394) has proposed a projection optical system that introduces light from a first focal point in an ellipsoidal mirror into a second focal point, accords the second focal point of the ellipsoidal mirror with a focal point of a parabolic mirror, and collimates light introduced into the focal point of the inclined parabolic mirror, obliquely projecting the collimated light onto a screen.
However, when plural optical surfaces are provided as in Japanese Patent Applications, Publication Nos. 2001-222063, 2001-221949 and 2000-104095, light's different entrance and exit gates need plural openings for the light to pass through, and thus more and longer spacing among mirrors by the openings, disadvantageously enlarging the optical system.
The projection optical system disclosed in Japanese Patent Application, Publication No. 07-013157 generates aberration since rays other than a principal ray do not pass through the first focal point in the ellipsoidal mirror. The same ray generates aberration at the second focal point in the ellipsoidal mirror, and an enlarged image incident upon the parabolic mirror increases the aberration on the screen, remarkably deteriorating imaging performance.
The conventional optical system that uses plural optical reflective surfaces requires a retainer mechanism that has at least two clearances for light to pass through, and thus has a difficulty in maintaining strength.